Measuring the Material Properties Beneath Geothermal Fields Using Interferometry

Abstract

Microseismicity provides a direct means of measuring the physical characteristics of faults and fractures beneath geothermal sites. Hundreds to thousands of small earthquakes often occur during operations. This seismicity helps define the active zone, allowing us to measure structural properties of the geothermal field. Here, we focus on two methods of seismic interferometry, ambient noise correlation (ANC) and the virtual seismometer method (VSM). ANC is based on the observation that the Earth's background noise includes coherent energy, which can be recovered by observing over long time periods and allowing the incoherent energy to cancel out. The cross correlation of ambient noise between a pair of stations results in a waveform that is identical to the seismogram that would result if an impulsive source located at one of the stations was recorded at the other, the Green function (GF). The calculation of the GF is often stable after a few weeks of continuous data correlation, any perturbations to the GF after that point are directly related to changes in the subsurface and can be used for 4D monitoring. VSM is a style of seismic interferometry that provides fast, precise, high frequency estimates of the GF between earthquakes. VSM illuminates the subsurface precisely where the pressures are changing and can monitor the evolution of the pressure field over time. With hundreds of earthquakes, we can calculate thousands of waveforms. At the same time, VSM collapses the computational domain, often by 2-3 orders of magnitude. This allows us to do high frequency 3D modeling. These methods allow us to estimate material properties, including absolute seismic velocities, Poisson's ratio, and seismic attenuation. When sufficient data are collected, we can also estimate changes in those properties over time.